Handling solid waste is an increasingly difficult problem in the industrialized nations. Sixty-one percent of the United States' solid waste is dependent on disposal in landfills. However, nationwide the number of solid waste landfills is falling as the landfills fill to capacity and close, while new landfills cannot be opened due to regulations. The national average for solid waste disposal fees has increased by about 400 percent since 1985. Such fees, called tipping fees, can be expected to raise an average of seven percent per year. In addition, the number of solid waste transfer stations has increased due to the closure of many landfills and the permitting of larger solid waste landfills.
These and other issues pertaining to the environmental impact that landfills will have in the future have prompted solid waste managers to seek methods to reduce volumes and disposal costs of municipal solid waste (MSW). Some experts contend that there is no waste volume problem, but there exists a sorting and recycling problem.
There have been a number of patents for treating and/or reprocessing MSW. These often involve some form of heat and pressure, and often a reaction vessel that may rotate. Although MSW varies in composition, there is a certain norm to its content. Other forms of waste have a biological, fiber, or cellulosic component, and can be processed in the same process that works for MSW or other biomass bearing material. These include sewer sludge, industrial waste streams, industrial byproducts, agricultural waste and byproducts, food processing waste and byproducts, and other sources. Cellulosic material represents approximately seventy percent of the bulk of typical MSW. Other terms for this fraction of MSW have been used, including putricible, organic, and biomass. These terms are considered interchangeable and it is to be understood that the use of the term “biomass bearing material” in this application encompasses MSW as well as all the waste or byproduct streams listed above.
The putrescible, biomass, organic, or cellulosic fraction of biomass bearing material includes all portions that are organic, which could eventually decay in a landfill and the decay could eventually lead to the production of methane gas and leachate. If this cellulosic material can be separated into reusable products, much of the pollution resulting from the disposal of organic bearing waste will be alleviated. Products that can be made from the cellulosic fraction of biomass bearing material include loose or palletized fuel or feedstock for gasification, bio refineries and conversion into ethanol and other chemicals and fuels. The biomass bearing fraction of MSW could also serve as a source of hydrogen and other liquid, elemental or gaseous products achieved through gasification or other thermal processes, including hydrolysis.
Once the cellulosic material is removed, the biomass bearing material is much reduced in volume and other recoverable fractions of the waste stream can be recovered. Ferrous and non-ferrous metals, plastics, and textiles can represent an additional fifteen percent of the remaining biomass bearing material.
There are processes available that can recover each of these materials and send them to their respective market. If the cellulosic and recoverable waste streams are removed from biomass bearing material, the volume of such waste streams going to landfills is typically less than fifteen percent of the original amount. This reduction in volume reduces hauling costs, landfill space requirements. Removal of the cellulosic portion of biomass bearing material also accomplishes this reduction while reducing effluents, including methane and leachate. Further, if the cellulosic portion is removed from the biomass bearing material, the remaining fraction of inorganic waste could be disposed of in waste sites reserved for construction or other inorganic waste, which is space that is often less regulated and therefore less expensive.
Many prior art reaction vessels for treating biomass bearing material utilize fairly high-pressure steam in order to cook and soften the cellulosic fibers of the biomass bearing material. This presents a problem because high-pressure steam vessels require significant licensing and inspection regimes, and must be built to withstand higher pressure. If the reaction vessel can achieve adequate softening, pulverization, and separation of cellulosic fibers while utilizing low pressures, the vessel could be lighter, less expensive to construct and operate. A lower pressure reaction vessel that achieves adequate softening, pulverization, and separation of cellulosic fibers is thus needed.
What is also needed is a reaction vessel that incorporates condensation of gaseous affluent steam from the reaction vessel. This condenses steam from the vessel, and also reduces or eliminates unpleasant gas emissions from the vessel.
There is also a need in the industry to provide a vessel and processing system that results in a product with fairly low moisture content, provides a sterilization effect upon the biomass bearing material, imparts a chemical change to the biomass bearing material and imparts beneficial handling, flow, and chemical characteristics to the product of processing in the reaction vessel, which thus converts the biomass from a waste into a feedstock for subsequent processes. Additionally, the non biomass fractions can benefit by removing labels from containers, fracturing glass for possible later separation from the waste stream, compacting and agglomerating plastics, and compacting ferrous and non-ferrous materials.